. >> reporter: after the explosions, cell phone service in the surrounding areas was completely shut down, to prevent any possible remote detonation. but they are still unclear how the bombs were set off. president obama promised federal help for victims and their families. >> any responsible individuals, any responsible groups, will feel the full weight of justice. >> reporter: sources tell cbs news that a man is being detained and police are questioning him. but he is not being called a suspect. investigators are also looking at surveillance tape and other images of the scene before and after the explosion to try to piece together what happened and who might be responsible. now, in response to what's played out here in boston, other major cities are now beefing up their own security. in fact, london is slated to host its own marathon this coming sunday. they are now taking a second look at their security. reporting live in boston. back to you. >> investigators are still coming every square inch of boston. a senior u.s. intelligence official says as many as two unexploded bombs turn